1. Technical Field
The present application relates to a hearing aid fitting technique. More specifically, it relates to a system, method, and program for determining the sound gain of a hearing aid, during hearing aid fitting.
2. Description of the Related Art
Hypacusia means difficulty in hearing sounds of a specific frequency or a frequency band, for example. The frequency or frequency band which presents hearing difficulty differs from user to user.
A hearing aid amplifies sounds so that the user can hear the sounds. The necessary amount of amplification differs for each user because auditory characteristics are different from user to user. In the present specification, “gain” means an amount by which a sound is to amplified (amount of gain).
Prior to beginning use of a hearing aid, fitting is conducted to determine a gain for each sound frequency. In order to perform appropriate fitting, it is necessary to accurately measure the auditory characteristics of a user.
In an auditory characteristics test, a hearing threshold level (HTL) is determined first. Next, an uncomfortable level (UCL) is determined. The HTL and the UCL are used for determining a dynamic range of sound pressure for sounds to be output from a hearing aid.
It is also possible to measure UCL by using electroencephalogram. For example, Thornton, A. R. et al, “The objective estimation of loudness discomfort level using auditory brainstem evoked responses”, Scandinavian Audiology, Vol. 16, No. 4, P. 219-225, 1987 (refereed to as Non-Patent Document 1 hereinafter), discloses that there is correlation between a V wave latency of an auditory brainstem response (ABR) to a click sound and the UCL.
Currently, several fitting theories are proposed in order to derive a gain for each frequency of a hearing aid or for each sound pressure of an input sound. For example, in the half-gain method, the gain of each frequency is made half of the HTL for that frequency. Berger's method slightly augments the amplification from 1000 Hz to 4000 Hz by taking into consideration the frequency band of the conversational voice. The POGO method reduces the gains at 250 Hz and 500 Hz by 10 dB and 5 dB, respectively. The NAL-R method performs amplification so that a frequency of long-term sound analysis of words will fall around a comfortable level.